The present invention relates to a toner for developing latent electrostatic images comprising large heat-fixing base particles in the surface of which small organic polymer particles are embedded.
As is generally known, a toner for developing latent electrostatic images formed on an electrophotographic photoconductor, electrostatic recording medium and the like essentially comprises a thermoplastic resin such as styrene resin and acrylic resin, with the further addition of pigments and magnetic powder if necessary. Generally in a dry-type development method, a toner having an average particle size of 5 to 25 .mu.m, with a low softening point, is used because of good image fixing capability, particularly at low temperatures. However, such kinds of toner particles with a low softening point are susceptible to mutual aggregation, or so-called blocking in their preservation or use, particularly in a high temperature atmosphere.
With the objective of eliminating such shortcomings, a toner composed of a blend of large particles with a low softening point and small particles with a high softening point has been proposed. However, in the case of this blend-type toner, shortcomings are indicated as follows:
(1) A sufficient blocking resistance is not maintained because of the easy contact of the large toner particles with one another, particularly when the amount of small particles is small compared with that of the large particles.
(2) The large particles are crushed to form small particles by contact with the carrier in the copying process thus forming a spent toner and consequently, low durability and degrading performance of the toner due to the film forming (so-called filming) on the photoconductor and on a carrier, if any, occur.
(3) A trend by which large toner particles are consumed in preference to the small toner particles during the copying process induces a change of the toner composition from the initial developing condition, reducing the quality of the images and the fixing properties.